Method of reducing corrosion



Patented Mar. 1, 1949 nm'rnon or aanuomc poaaosrou John 0. Almen, RoyalOak, Mich., assignor to' General Motors Corporation, Detroit, Mich, a

corporation of Delaware No Drawing.

1 Shot peening or blasting a surface with smooth steel balls forcompressively stressing the skin of a metal article increases materiallyresistance to fatigue failure. peening can be effected for the purposeof resisting corrosion of several kinds. a magnesium strip one-sixteenthof an inchv in thickness stressed by bending will fail by stresscorrosion in approximately two minutes time when immersed in a saltsolution. On the other hand the same specimen will endure indefinitelyafter the surface has been compressively stressed by light shot peeningin the manner to be described.

Magnesium or other similar metals when closely fitted to a dissimilarmetal, such as iron, are likely to corrode badly. Corrosion which occursbetween bolted surfaces, onsplines, press fits and other like closelyfitted parts is known under the general name of fretting corrosion. Suchcorrosion is retarded and its penetration reduced by compressivelyprestressing the surface. Stress corrosion is severe in magnesium'andits avoidance is of special importance in aircraft construction largelycomprised of magnesium metals in critical parts.

Rapid electrolytic corrosion occurs on such metals as aluminum,magnesium, etc., when iron dust is retained on the surface of the metalfrom shot blasting. Such corrosion occurs on exposed surfacesparticularly .when immersed in salt solutions although it will occur inthe atmosphere particularly in damp weather, such as rusting of steelparts. 7

Stress corrosion is the more rapid attack of normal atmosphericcorrosion. The penetration is rapid, presumably because when a tensionstress is present the gaps formed'by corrosive severance of the bonds ofadjacent particles will This corrosion results from very small motionsbetween the tightly fitted parts. It may fundamentally be anelectrolytic action since we'are told that all corrosion iselectrolytic, but electrolytic action may occur without the presence ofI- have discovered that proper For example,

Application January 5, 1944, Serial No. 517,102

'3 Claims. (01. 29-148) metal between regions of dissimilar stress.Fretting corrosion, therefore, is not dependent-on the contamination dueto shot peening whether the fretting corrosion occurs between similar ordissimilar metals.

Compressively prestressing the surface of parts 7 subject to corrosiondamage has proved itself highly effective.

However, care must be exercised to see that the metal surface to becompressed is free from contact with a dissimilar metal during theprestressing operation including dust or flake particles from the shotwhich tend to accumulate at the shot blasting station and to settle onarticles being operated upon and under the hammering to be embedded inand to contaminate the metal surface. Thus iron dust from steel shotwould contaminate the surface of a magnesium article and must be removedor prevented from' adhering in order to avoid subsequent electrolyticaction between the dissimilar metals which electrolytic action wouldundo the effect of prestressing in reducing corrosion.

Magnesium and similar materials corrode very rapidly when stressed intension and less rapidly when unstressed. The object of compressiveprestressing is to place a thin surface layer of the material in a stateof compression even though underlying layers are stressed in tension.

To accomplish the result, I have used the expedient 'of coating thesurface of the article to be treated with a strong paint or lacquer andthen after the coating has dried and set, peening through the coating. Astrongly adherent and elastic paint is required to avoid flaking duringthe peening operation but the coating should be thin so as not to undulycushion the impact 01' the shot. Thus the impacts are transmitted to themetal surface by means of a nonmetallic agency and the surface ismaintained free from contamination. Once the surface has been densifledthe paint, if desired, can be removed and at dissimilar metals. Burns ofthe Bell Laboratories states that electrolytic action may occur in a anyrate the magnesium may be joined or press fitted to an iron part withoutfear of serious corrosion problems. plish the same result in the absenceof contact between dissimilar metals, it is proposed to use smooth shotof a nonmetallic material, such as glass or a relatively strong plastic.

Glass shot has been found to be satisfactory for this purpose.

In my study of the problem I have noted resistance'to other forms ofcorrosion results-from shot peening. Fretting corrosion has been great-This has As an alternative to accomdue to the surface roughness of thepeened specimen but penetration of corrosion is apparently greatlyreduced. Similar results-have been obtained in fretting corrosionbetween dissimilar metals in airplane engines.

In addition to shot peening, the results described abo've may beobtained by other prestressing means such as rolling with small rollersor balls. It is only necessary that the surface the metal be stressed incompression.

I claim:

1. The method of reducing surface corrosion between tightly fittedpartsof dissimilar metals, comprising subjecting one ofthe parts, beforethey are tightly fitted, to a conditioning treatment which includes prehating the joint surface with a thin elastic film capable of transmittingimpact therethrough and incapable of electrolytic action with thematerial of said surface and compressively stressing the surface beneathsaid film by repetitive blows of an impacting body against the filmcovered skin of the joint surface which precompresses the film coveredskin of the joint surface and thereafter tightly fitting together saidparts with the joint surfaces of one in contact with the precompressedskin of the joint surface of the other. Y

2. The method of decreasing the tendency toward surface corrosion inarticles containing a magnesium metal, including applying on the surfaceto be protected, a coating of strongly adherent and elastic paint,allowing the paint to set and then peening the metal through the coatingto stress the surface of the metal in compression.

5 uniformly spherical shot.

3. The method of prestressing in compression the surface of an articleformed of largely magnesium including peening said surface by blastingthe same with nonmetallic smooth and JOHN O. ALMEN.

file of this patent:

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Metals Handbook, 1939 'ed., Am. Society for Metals, Cleveland, Ohio,page 1614.

